The present invention relates to a current resonance DC-DC converter that is configured with a switching circuit, a resonance circuit and an AC-DC conversion circuit. Specifically, in the current resonance DC-DC converter, the switching circuit and the resonance circuit are connected to a primary winding of a transformer of the current resonance DC-DC converter. Further, the AC-DC conversion circuit is connected to a secondary winding of the transformer of the current resonance DC-DC converter. Further, the switching circuit has a bridge circuit configuration that has at least a pair of first switches that alternately perform ON and OFF operations. The resonance circuit generates a resonance current in the primary winding of the transformer. The AC-DC conversion circuit converts an AC voltage that is induced in the secondary winding of the transformer into a DC voltage and outputs the DC voltage.
As this kind of DC-DC converter, Japanese Publication Number JP H08-289540 discloses a DC-DC converter that is used in a switching power supply device. In this DC-DC converter, a first capacitor for smoothing (also referred to as “a smoothing capacitor” below) that works as a DC power source of this circuit is connected between a pair of power supply terminals. Further, a series circuit of first and second switches (also referred to as “primary side switches” below) is connected to the smoothing capacitor in parallel. In this case, a pair of the primary side switches is composed with an insulated gate (a metal oxide semiconductor (MOS)) field effect transistor (FET). Thus, the pair of the primary side switches respectively includes a control switch that corresponds to an original FET function and a diode connected to the control switch in reverse parallel. Further, a capacitor for forming a partial resonance circuit is respectively connected to each primary side switch in parallel.
Further, a series circuit (an output resonance circuit) of the primary winding and a resonance capacitor that have a resonance inductance is connected between a node of the primary side switches (a connection middle point therebetween) and a lower end of the smoothing capacitor, i.e., a source of one of the primary side switches located on a low potential side (that is, the second switch). Further, the primary winding of the transformer has an excitation inductance that is equivalently connected to the primary winding in parallel in addition to an inductance that is composed with a leakage inductance.
Further, the secondary winding of the transformer is divided into first and second (secondary) windings by a center tap. One end of each is respectively connected to one end of an output smoothing capacitor via third and fourth diodes. The center tap is connected to the other end of the output smoothing capacitor. Further, a pair of output terminals for connecting a load (not shown) is connected to the output smoothing capacitor. A control circuit for making each of the primary side switches perform alternately ON and OFF operations makes an output voltage constant by changing ON and OFF frequencies of each of the primary side switches according to a change of an input voltage (a charging voltage for the smoothing capacitor) or an output voltage (a charging voltage for the output smoothing capacitor).
In this DC-DC converter, in a case in which the smoothing capacitor is already charged, when the primary side switch located on a high potential side of the pair of the primary side switches (that is, the first switch) is in an ON state, an electric current flows in a series resonance circuit, which is a closed circuit, by a series resonance. The series resonance circuit is configured with the smoothing capacitor, the primary side switch in the ON state mentioned above (the first switch), the primary winding and the resonance capacitor. Further, when the primary side switch located on the low potential side of the pair of the primary side switches (that is, the second switch) is an ON state, an electric current flows in a series resonance circuit, which is a closed circuit, by a series resonance. The series resonance circuit is configured with the resonance capacitor, the primary winding, and the primary side switch in the ON state mentioned above (the second switch). In this way as explained above, the series resonance circuit that is configured with the leakage inductance of the primary winding and the capacitor is driven by the ON and OFF operations of each of the primary side switches. As a result, output power that corresponds to an electric current (that is, electric power) generated by the series resonance is obtained at the secondary winding of the transformer. The DC-DC converter that has the configuration explained above corresponds to an LLC current resonance converter. The control circuit controls an output voltage as a constant by changing ON and OFF frequencies (by a frequency control) of the first and second switches within a frequency range in which output power greatly changes when a frequency is changed.
The conventional DC-DC converter described above, however, still has some problems to be solved. As an example, a case in which the primary side switch located on the low potential side (the second switch) is driven so as to be in an ON state in the DC-DC converter is explained. After an ON period of the primary side switch located on the high potential side (the first switch) is finished, during a dead time period that is defined between a time that the ON period of the first switch is finished and a time in which the primary side switch located on the low potential side (the second switch) is driven to be in the ON state, a capacitor that is connected to the primary side switch located on the low potential side (the second switch) in parallel is discharged (that is, a voltage between both ends of this primary side switch becomes zero volts) in accordance with an electric current that flows by a partial resonance in a path that includes the capacitor which is connected to the primary side switch located on the low potential side in parallel, a leakage inductance of the primary winding, and an excitation inductance. As a result, preparation for a zero volt switching of the primary side switch located on the low potential side is completed.
However, a current amount of the electric current, which flows by the partial resonance in the dead time period, depends on both a transmission electric current and an excitation current. The transmission electric current is transmitted from the primary winding to the secondary winding of the transformer. The excitation current flows in the excitation inductance of the primary winding. Specifically, the current amount of the electric current that flows by the partial resonance increases as a total of each current value of the transmission electric current and the excitation inductance is larger. Therefore, when a percentage of the transmission electric current for the excitation current is large (when the current amount of the electric current that flows by the partial resonance depends on a current value of the transmission electric current), the decreased current amount of the electric current that flows by the partial resonance is larger because the transmission electric current decreases under a light load. As a result, the following problem occurs: the capacitor that is connected to the primary side switch located on the low potential side in parallel cannot be adequately discharged so that the preparation for the zero volt switching of the primary side switch cannot be completed.
Further, the current amount of the electric current that flows by the partial resonance does not depend on the transmission electric current when the current value of the excitation current is large. For achieving that state, the excitation inductance needs to be decreased. As a result, it is necessary to provide a gap at a core of the transformer. However, when the gap is provided at the core, a radiation noise is generated because the magnetic flux expands outwardly from the core around the gap or an overcurrent loss is generated at the winding because the magnetic flux that expands outwardly crosses the winding that is wound around the core because the magnetic flux expands outwardly from the core around the gap. Each problem explained above similarly occurs even when the primary side switch located on the high potential side (the first switch) is driven to be in the ON state.